The invention relates to particular mixtures of polyisocyanates which are blocked with 3,5-dimethylpyrazole, and to the use thereof as crosslinking component in single-component stoving lacquers, especially automotive clear lacquers.
The formulation of OH-containing polyacrylate resins with blocked polyisocyanates or with mixtures of melamine resins and blocked polyisocyanates to form binders for xe2x80x9csingle-componentxe2x80x9d stoving lacquers is state of the art. The properties of lacquers produced therefrom are determined by the nature both of the polyisocyanate component and of the blocking agent, since it is known that a certain proportion of the latter always remains in the lacquer.
3,5-Dimethylpyrazole, DMP for short, is mentioned as an NCO-blocking agent in stoving lacquers for the first time in EP-A 0159117B1. A comparison of the blocking agents butanoneoxime, 1,2,4-triazole and DMP is described in Farbe and Lack 7/96, 102nd year, p. 51-58, (Engbert et al.). That study shows that DMP has advantages over butanoneoxime as regards stoving temperature and thermal yellowing resistance. It has also been found that the resistance to acids of a clear lacquer based on a DMP-blocked IPDI polyisocyanate is very good and is exceeded slightly only by the analogous two-component lacquer (without blocking agent). No mention is made of the scratch resistance of that clear lacquer.
Resistance to acids and to scratching are important criteria of an automotive clear lacquer as the uppermost layer, which must withstand, for example, the influence of the surrounding atmosphere and also the conditions of a car wash. In the study cited above, a clear lacquer in which 1,2,4-triazole is used as blocking agent exhibits markedly poorer properties, for example in respect of resistance to acids.
In the publication European Coatings Conference, Berlin 1999, Vincentz Verlag, xe2x80x9cBlocked Isocyanatesxe2x80x94xe2x80x9cMethods of Evaluation and Recent Developmentsxe2x80x9d, I. Rimmer et al., Baxenden Chemicalsxe2x80x9d, information is given mainly regarding de-blocking temperatures for DMP-blocked isocyanates. The question of how to produce, using a DMP-blocked polyisocyanate, a clear lacquer having both good scratch resistance and good resistance to acids is not discussed, however.
The object of the invention is to develop blocked polyisocyanates which can be processed to a stoving lacquer having
1. good resistance to acids
2. good scratch resistance
3. good resistance to thermal yellowing and
4. a relatively low stoving temperature of from 130 to 150xc2x0 C. (at 30 minutes).
That object could be achieved using the blocked polyisocyanate mixtures according to the invention.
The invention provides mixtures of blocked polyisocyanates containing
a) blocked polyisocyanates based on 1,6-diisocyanatohexane (HDI),
b) blocked polyisocyanates based on cycloaliphatic diisocyanates, and
c) 3,5-dimethylpyrazole as the sole blocking agent for the polyisocyanates mentioned under a) and b), characterised in that the blocked polyisocyanates are present in relative proportions a):b)=1:1.8 to 2.2.
The invention relates also to the use of the blocked polyisocyanate mixtures according to the invention in admixture with melamine resins in the hardening of stoving lacquers, characterised in that up to 50 wt. %, based on total hardener component, of the blocked polyisocyanate mixtures a)+b) are used concomitantly.
Finally, the invention relates to the use of the blocked polyisocyanate mixtures according to the invention as hardener component in single-component stoving lacquers, especially for automotive clear lacquers.
The mixtures according to the invention are a combination of the flexible blocked polyisocyanate component a) and the hard blocked polyisocyanate component b), the mixing ratios varying within the range from a:b=1:1.8 to a:b=1:2.2. A mixing ratio of a:b=1:2 is preferred.
There come into consideration as polyisocyanates for component a) the lacquer polyisocyanates, known per se, that are based on 1,6-diisocyanatohexane (HDI) having an NCO content of from 19 to 25 wt. % and that contain biuret, isocyanurate, allophanate, iminooxadiazinedione (asymmetric trimer), urethane and/or uretdione groups. Preference is given to the HDI-based lacquer polyisocyanates containing predominantly isocyanurate groups, trimerised HDI for short.
There come into consideration as polyisocyanates for component b) the lacquer polyisocyanates, known per se, that are based on cycloaliphatic diisocyanates having an NCO content of from 12 to 23 wt. % and that contain isocyanurate, allophanate, uretdione and urethane groups. Examples of cycloaliphatic diisocyanates are 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl-cyclohexane (IPDI), bis-(4-isocyanato-cyclohexyl)-methane (H12 MDI or Desmodur W(copyright)/Bayer AG), 2,6- and/or 2,5-bisisocyanato-norbornane or 1,4-bisisocyanatomethyl-cyclohexane. It is also possible to use 1:1 mixtures of polyisocyanates based on IPDI and Desmodur W(copyright). Preference is given to lacquer polyisocyanates based on IPDI and Desmodur W(copyright) and containing predominantly isocyanurate groups. A lacquer polyisocyanate based on trimerised IPDI is a solid resin having an NCO content of approximately 17 wt. % and accordingly is available commercially as a 70% solution having an NCO content of approximately 12 wt. %. A polyisocyanate based on Desmodur W(copyright) and containing isocyanurate groups is described in Example 6.
The NCO-blocking agent 3,5-dimethylpyrazole is a colourless solid (m.p. 107xc2x0 C.), which can be obtained relatively easily and quantitatively by condensation of 1 mol of acetylacetone with 1 mol of hydrazine hydrate. However, for blocking of the isocyanates it is also possible to use the warm reaction solution of the mentioned starting materials, as described, for example, in EP-A 0713871B1. According to the invention, all the isocyanate groups of the isocyanates used are blocked, so that no free isocyanate groups are present in the mixtures according to the invention.
The blocked polyisocyanate mixtures according to the invention are mixed with organic polyhydroxyl compounds in a conventional manner to form single-component lacquer binders, by combining the amount of blocked NCO groups with the corresponding amount of OH groups. The stoichiometric ratio of blocked NCO:OH=1:1.
If a single-component stoving lacquer is crosslinked predominantly, for example, with melamine resin and the blocked polyisocyanate mixture according to the invention is used as modifying component, then the mixing ratio of the binder components must be determined empirically. A reference point for the quantitative distribution of polyacrylate:melamine resin:blocked polyisocyanate mixture is provided by the ratio of 70:20:10 wt. %, based on the particular solid resins in question, indicated in the Examples. In the case of such mixed crosslinking, the crosslinking components, namely melamine resin and blocked polyisocyanate mixture, are present in a weight ratio of 2:1. Depending on the desired effect, that is to say whether melamine resin crosslinking or crosslinking with blocked polyisocyanates is preferred, the above mixing ratio may be varied from 5:1 to 1:2.
Using the blocked polyisocyanate mixtures according to the invention, clear lacquers having good resistance to acids, scratch resistance and thermal yellowing resistance are obtained.